Infrared spectrometers, such as Fourier transform infrared (FTIR) spectrometers, dispersive-type infrared spectrometers and filter-based infrared spectrometers, are widely used for measuring the chemical composition and characteristics of materials. In an infrared spectrometer, infrared radiation provided by a source having a relatively broad emission bandwidth is passed through a spectrometer, and then through a sample, before reaching a suitable detector. The information obtained from the detected radiation is analyzed to determine information concerning the sample, for example to determine an absorbance spectrum of the sample in the infrared range. This spectrum may then be used to identify the chemical composition of the sample.
It is desirable that the infrared source for an infrared spectrometer have a relatively broad and uniform emission spectrum over the wavelengths of interest. Unfortunately, conventional broadband infrared sources that meet these criteria typically possess several undesirable characteristics, including relatively high power consumption and the production of relatively high levels of stray infrared radiation, e.g., radiation that travels in unwanted directions. These characteristics are undesirable because they produce a substantial amount of wasted energy and an unwanted heating of the parts of the spectrometer.
Unwanted heating of spectrometer components as a result of radiation from the source propagating in unwanted directions has particularly been a problem with conventional infrared sources. An efficient infrared source will, by definition, produce large amounts of infrared radiation. The electrical filaments which are heated by electrical current to provide the infrared typically radiate in all directions, so that much of the infrared radiated from the filament does not go into the useful beam that is directed into the spectrometer, even if a reflector is used adjacent to the source to help form the beam. Consequently, a significant amount of infrared wavelength energy is not accounted for by the beam and will be absorbed by adjacent structure within the spectrometer, heating up the spectrometer. Moreover, any stray infrared from the source may interfere with the signal reaching the infrared detector, and therefore the detector must be carefully shielded from the source. Even if a conventional infrared source is surrounded by heat insulation, the insulation itself tends to increase in temperature and will eventually itself become a source of heat which is transmitted either by conduction, convection or reemission of infrared radiation to other parts of the spectrometer. Thus, conventional infrared sources have been both a heat load on the spectrometer and also a significant power drain which adds to the total power requirements of the spectrometer.
An example of an infrared source commonly used in spectrometry is the "glow bar," which is a coil of silicon carbide that is resistively heated to a temperature of about 1200 degrees Celsius. Due to the relatively large size of the coil, this type of source is relatively costly and inefficient. For instance, the glow bar consumes about 150 watts of power, which requires a relatively large power supply, and usually one that is separate from the power supply for the electronic components in the spectrometer. The combination of glow bar and power supply adds cost, weight and size to the spectrometer. The glow bar also radiates a lot of heat--more heat than can be passively dissipated inside a modern spectrometer. To remove the excess heat, the glow bar is typically cooled with water flowing through a water jacket that surrounds the source. Water cooling adds plumbing and water supply requirements, which adds further cost, weight, size and complexity to the spectrometer. Manufacturing and field service are also more difficult, since the water jacket can interfere with access to the source.
Infrared spectrometers for special purposes may sometimes be used in environments where the spectrometer may be exposed to corrosive gases. In such environments, the hot source element (commonly operating at about 1200.degree. Celsius) cannot be exposed to these gases. Similarly, it would not be acceptable for any structures near to the source to be allowed to reach temperatures sufficiently hot to accelerate the corrosive effects of the gases.
Infrared spectrometers often use a purge gas such as dry air or nitrogen to pressurize the spectrometer to reduce corrosion caused by ambient gases that might react with the source electrode. Many manufacturing plants use gases, for example Freon-113, that will react with the hot source electrode or hot exposed parts to produce very corrosive chemicals. For example, only a few parts per billion of Freon-113, when exposed to a 1200.degree. C. source, can produce a small but significant quantity of various acids, including hydrochloric acid and hydrofluoric acid. These acids corrode the source electrode and can also condense on the cooler components in the interferometer, for example, the fasteners, flex pivots, and electrical connectors. Purge gas failures can thus result in damage to the spectrometer.